Python (programming language)

Python is a dynamic object-oriented, general purpose interpreted programming language which runs on many different computer platforms and mobile devices. Python is open source software and is published under an OSI-approved license. Python aims to be a language that is efficient, coherent, readable, and fun to use. Because Python is an interpreted language, Python programs run immediately without the need for lengthy compile and link steps.

History

Python was first published by Guido van Rossum in pre-release (early-adopter) form in 1991, and to this day he remains the project leader and final arbiter of Python Enhancement Proposals (PEPs).

Python 2.5.1 is the current production release, and is very stable.

Python's major (standard) releases were:

• Python 2.5.1 (April 2007)
• Python 2.4.4 (October 2006)
• Python 2.3.6 (November 2006)
• Python 2.2.3 (May 2003)
• Python 2.1.3 (April 2002)
• Python 2.0.1 (June 2001)
• Python 1.6.1 (September 2000)
• Python 1.5.2 (April 1999)
• Python 1.4 (October 1996)
• Python 1.3 (October 1995)
• Python 1.2 (April 1995)
• Python 1.1 (1995)
• Python 1.0 (1994)

Python was originally coded in C for maximum portability, and that is still its reference (canonical) implementation, known as Classic Python (CPython), but the Python programming language has now also been ported to the Java virtual machine (as Jython) as well as Microsoft's .NET CLR virtual machine (as IronPython).

Under development is a full refactoring for Python (version 3.0), which is known as Python 3000 (Py3k) and which is currently in early alpha (pre-production) form.

Examples

Hello World

The code for the "hello world" program can hardly be simpler:

print 'Hello World'

This can be put in a file "hello.py", say, and executed with

python hello.py

from the usual command line of the operating system. Alternatively, the code can be typed directly in an interactive Python environment (Python command line interpreter or IDLE, which make parts of a standard Python distribution).

Calculator

Python interpreter invoked from the command line gives an easy access to a scientific calculator. At prompt (denoted here by >>> ) one types

>>> 2+3*(1+1)

to get 8 in the result. Division of integers returns integer result, so

>>> 7/2

is 3 (floor from the exact result). If a real result is needed, then at least one ingredient should be 'real', as below

>>> 7.0/2

More interesting functions may be found in the math module. This is how to use it.

>>> from math import *
>>> print sin(pi/2)

High quality graphs may be obtained with matplotlib library

Files

A useful Python construction is related to working with files. Line-by-line Perl-like file processing can be realized with a standard for loop as in the following simple word count script. Below, as always in Python, comments start with the hash '#' sign and continue to the end of line.

(char_count,word_count,line_count)=0,0,0      # multiple assignment is OK
file = open ('myfile.txt')                    # standard opening of the file
for line in file:                             # this is "idiomatic" use of of the "for" loop
  wordlist = line.split()                     # splitting line into the list of words
  word_count += len(wordlist)                 # counting words
  line_count += 1                             # counting lines
  char_count += len(line) - 1                 # counting characters
print line_count, word_count, char_count      # we're done

Notice the indentation that indicates what code is executed within the loop. This is the standard Python notation, a part of the syntax. When the end of file is reached, the loop terminates and the results are printed by the last line of the code. Note also that the variable line contains the terminating newline character, so that 1 is subtracted from its length for the character counting. Another interesting observation can be made: the method (i.e. function) of splitting a string is 'provided' by this very string. Indeed, Python is an objective language, the string variables are objects and related methods are its attributes.

Internet access

The following script counts the images on the Citizendium Main Page.

import urllib2                      
cnt=0
for line in urllib2.urlopen('http://en.citizendium.org/wiki/Main_Page'):
  cnt += line.count('<img src')
print cnt

First, we import a module from the standard library. The latter is said to be rich and often regarded as Python's strong point. In fact, Python official pages declare a "batteries included" philosophy. Now, the source HTML files of web pages can be treated much like the local files, e.g. processed line-by-line with a for loop. In the above example the variable line is a string containing a piece of HTML code of Citizendium's Main Page. Embedded images are inserted in this code with a text that begins with '<img src'. So it's enough to count instances of this last string. The appropriate method count() is at hand in the variable line.

Syntax

Remarkable global features of the Python syntax include high readability of the code, which is not independent of the use of indentation to separate blocks of code and a general "one statement per line" principle.

Implementations

Python's official distribution is known as CPython. It's written in C and functions as a virtual machine interpreting bytecode-compiled Python programs. Jython is an implementation for the Java Virtual Machine, which can run either standalone (like CPython) or as an embedded scripting engine. IronPython is an implementation for the Common Language Runtime (.NET and Mono). PyPy is an implementation written in Python that targets several backends, including C, LLVM, JavaScript, JVM and CLR.

See also

• IDLE, the Integrated Development Environment for Python

Books

• Beazley, David M. Python Essential Reference, 3rd Ed. Sams, 2006 ISBN 0672328623
• Chun, Wesley J. Core Python Programming, 2nd Ed. Prentice Hall, 2006 ISBN 0132269937
• Goerzen, John. Foundations Of Python Network Programming Apress, 2006 ISBN 1590593715
• Hetland, Magnus Lie. Beginning Python: From Novice To Professional Apress, 2005 ISBN 159059519X
• Lutz, Mark. Programming Python, 3rd Ed. O'Reilly, 2006 ISBN 0596009259
• Lutz, Mark and Ascher, David. Learning Python, 2nd Ed. O'Reilly, 2003 ISBN 0596002815
• Martelli, Alex. Python In A Nutshell, 2nd Ed. O'Reilly, 2006 ISBN 0596001886
• Martelli, Alex. Ravenscroft, Anna. Ascher, David. Python Cookbook, 2nd Ed. O'Reilly, 2005 ISBN 0596007973
• Zelle, John M. Python Programming: An Introduction To Computer Science Franklin Beedle, 2003 ISBN 1887902996

External links

• Python Programming Language Homepage
• Guido van Rossum's Introduction to Python
• Richard Gruet's Python Reference Manual. Contains also a compilation of useful Python-related resources (see 'Front matter').
• Dive into Python
• PythonForum.org